There are currently over 20,000 MRI scanners performing over 60 million studies world-wide [The Nobel Prize in Physiology or Medicine. Press Release, Oct. 6, 2003. http://nobelprize.org/nobel prizes/medicine/laureates/2003/press.html]. About half of these scans are performed in the United States (US), where the number of scans has been doubling about every 3 years for the past decade (see FIG. 1A). In the US, over half of the existing scanners operate at 1.5 Tesla (1.5 T) [Bottomley P A, Hart H R, Edelstein, Schenck J F, Smith L S, Leue W M, Mueller O M, Redington R W. NMR imaging/spectroscopy system to study both anatomy and metobolism. The Lancet 1983; ii, 273-274], while most luminary institutions active in medical imaging research have added 3 T MRI systems, and 4 T [Hardy C J, Bottomley P A, Roemer P B, Redington R W. Rapid 31 P spectroscopy on a 4 Tesla whole-body system. Magn Reson Med 1988; 8: 104-109] and 7 T [Vaughan J T, Garwood M, Collins C M, Liu W, DelaBarre L, Adriany G, Andersen P, Merkle H, Goebel R, Smith M B, Ugurbil K. 7 T vs. 4 T: RF power, homogeneity, and signal-to-noise comparison in head images. Magn Reson Med 2001; 46: 24-30] whole body systems are also commercially available. While MRI is not considered a significant risk [Young F E (Food and Drug Administration). Federal Register, Mar. 9, 1988; 53: 7575-7579], it is not hazard-free.
The potential for radio frequency (RF) power deposition and heating in human MRI was realized as early as 1978 [Bottomley P A, Andrew E R. RF magnetic field penetration, phase-shift and power dissipation in biological tissue: Implications for NMR imaging. Phys Med Biol 1978; 23: 630-643]. The mechanism for heating is the induction of eddy currents in the body by the time-dependent RF magnetic field in accordance with Faraday's Law, due to the finite conductivity of the body. Models comprised of homogeneous cylinders of tissue of measured conductivity and dielectric constant, are solved analytically for various configurations. The key results are: (a) local peak and average specific power absorption rates (SARs) in W/kg or W/cm3 can be determined from the known RF pulse width, duty cycle, flip-angle, and sample size using relatively simple formulae [Bottomley P A, Andrew E R. RF magnetic field penetration, phase-shift and power dissipation in biological tissue: Implications for NMR imaging. Phys Med Biol 1978; 23: 630-643; Bottomley P A, Roemer R B. Homogeneous tissue model estimates of RF power deposition in human NMR studies. Local elevations predicted in surface coil decoupling. Annal NY Acad Sci 1992; 649: 144-159; Bottomley P A, Edelstein W A. Power deposition in whole body NMR imaging. Med. Phys 1981; 8: 510-512]; (b) SAR varies approximately quadratically with MRI frequency or field strength, and with sample radius at lower frequencies (<100 MHz); and (c) peak SAR is related to the average SAR by simple numerical factors [Bottomley P A, Roemer R B. Homogeneous tissue model estimates of RF power deposition in human NMR studies. Local elevations predicted in surface coil decoupling. Annal NY Acad Sci 1992; 649: 144-159].
That excess RF power deposition can cause heating and burns is evidenced by voluntary reports to the US Food and Drug Administration (FDA) of injuries received during clinical MRI, as summarized in the tabulation provided in FIG. 1 for 2005 and 2006 [U.S Food and Drug Administration, Center for Devices and Radiological Health, MAUDE data base reports of adverse events involving medical devices. (http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfMAUDE/search.CFM)]. Many of these injury reports are not linked to patient proximity to leads, metal, separate receiver coils, or the magnet bore, which may pose additional risk. The safety of RF exposure during clinical MRI is regulated via government and industry guidelines [Guidance for Industry and FDA Staff Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices. USA Food & Drug Administration (http://www.fda.gov/cdrh/ode/guidance/793.pdf), Jul. 14, 2003; Particular requirements for the safety of magnetic resonance equipment for medical diagnosis (IEC 60601-2-33:2002). European Committee for Electrotechnical Standardization Central Secretariat: rue de Stassart 35, B—1050 Brussels]. The FDA and International Electrotechnical Commission (IEC) guidelines adopted in Europe are summarized in Table 1.
TABLE 1IEC and FDA guidelines on SAR and heating for human MRI studiesIEC limitsWhole bodyHeadHead, trunkExtremities(6 min average)averageaveragelocal SARlocalNormal (all patients)2 W/kg (0.5° C.)3.2 W/kg10 W/kg20 W/kg1st level (supervised)4 W/kg (1° C.)3.2 W/kg10 W/kg20 W/kg2nd level (IRB approval)4 W/kg (>1° C.)>3.2 W/kg>10 W/kg>20 W/kgLocalized Heating limit39° C. In 10 g38° In 10 g40° In 10 gFDA limits4 W/kg3 W/kg8 W/kg In 1 g12 W/kg In 1 gfor ≧15 minfor ≧10 minfor ≧10 minfor ≧5 min
RF exposure also is a factor in assessing the safety of MRI in human research overseen by Institutional Review Boards (IRBs). Consequently, accurate RF dosimetry is central to the safe operation of thousands of MRI scanners and millions of human MRI scans. Indeed, issues relating to SAR are listed among the top three unsolved problems and unmet needs by each of three study groups of the International Society of Magnetic Resonance in Medicine [Unsolved Problems and Unmet Needs in MR: Results from a survey of ISMRM Study Groups, December 2005. (http://www.ismrm.org/07/UnsolvedProblems_Results.htm)], underscoring the need for providing accurate and independent methods of measuring it.
The IEC defines the whole-body SAR as the absorbed RF power (PA in Watts) divided by patient mass (m), and the partial body SAR is calculated based on the body mass in the coil which may be modeled by homogeneous cylinders [Particular requirements for the safety of magnetic resonance equipment for medical diagnosis (IEC 60601-2-33:2002). European Committee for Electrotechnical Standardization Central Secretariat: rue de Stassart 35, B-1050 Brussels]. The local SAR in 1 g or 10 g of tissue is determined from experimentally-validated models or by experiments on phantoms. For homogeneous cylinders and spheres, the models yield a local SAR 2-4 times average, while heterogeneous models with quadrature excitation yield ratios of 4.5-6 for the head at 63-175 MHz [Collins C M, Li S, Smith M B. SAR and B1 field distributions in a heterogeneous human head model within a birdcage coil. Magn Reson Med 1998; 40: 847-856; Collins C M, Liu W Z, Wang J H, Gruetter R, Vaughan J T, Ugurbil K, Smith M B. Temperature and SAR calculations for a human head within volume and surface coils at 64 and 300 MHz. JMRI 2004; 19: 650-656; Nguyen U D, Brown J S, Chang I A, Krycia J, Mirotznik M S. Numerical evaluation of heating of the human head due to magnetic resonance imaging. IEEE Trans Biomed Eng 2004; 51: 1301-1309], and 10-16 for male and female torsos at 1.5 T and 3 T [Simunic D. Calculation of energy absorption in a human body model in a homogeneous pulsed high-frequency field. Biolelectrochem Bioenerg 1998; 47: 221-230; Liu W, Collins C M, Smith M B. Calculations of B1 distribution specific energy absorption rate and intrinsic signal-to-noise ratio for a body-size birdcage coil loaded with different human subjects at 64 and 128 MHz. Appl Magn Reson 2005; 29: 5-18]. Thus, once an average SAR is determined via PA/m, a model-based local maximum can be obtained by simple multiplication by the corresponding model factor, at least for the above frequency ranges.
One simple method of measuring SAR that satisfies the IEC guidelines and does not require MRI parameters, determines PA from the incident (forward minus reflected) root-mean-square (rms) power PI at the transmitter coil. This method, however, does require that the coil's quality factors (Q) be measured with the subject load (QL), and without the subject load (QU) [Bottomley P A, Redington R W, Edelstein W A, Schenck J F. Estimating RF power deposition in body NMR imaging. Magn Reson Med 1985; 2: 336-349; Mansfield P, Morris P G. NMR imaging in Biomedicine. Academic Press, NY, 1982, p 313.20].
In this method, the amount of power deposited in the subject is determined using the following relationship:PA=PI(1−QL/QU).  [1]
Although determining these parameters at each exam would provide a straightforward approach to measuring SAR, various expediencies are commonly adopted for reporting and limiting SAR in many commercial clinical MRI scanners. For example, the Q's and PI are generally not measured at the coil terminals by the scanner during set-up for a patient study, particularly in older scanners. Instead, fixed factory-determined parameters (e.g., factory software parameters) characterizing the selected coil-type are often entered into scanner configuration files to calculate the SAR for a desired duty cycle, with conservative safety factors added to accommodate variations that may occur in the field. These “scanner SAR” values are utilized by the scanner for limiting pulse sequence parameters, and are generally accessible to the MRI operator.
The accuracy with which such “scanner SAR” values measure the true average SAR may be compromised when coil Q's and power losses in the transmit line change with time, and/or when such conservative safety factors, are introduced into the scanner's SAR calculation by the manufacturer. This uncertainty is further compounded by those cases where RF burns are sustained by patients undergoing MRI as illustrated in FIG. 1 [U.S Food and Drug Administration, Center for Devices and Radiological Health, MAUDE data base reports of adverse events involving medical devices, (http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfMAUDE/search.CFM); Dempsey M F, Condon B, Hadley D M. Investigation of the Factors Responsible for Burns During MRI. J. Magn. Reson. Imaging 2001; 13: 627-631]. Although such cases represent a tiny fraction of the millions of clinical MRI scans performed annually, they are direct evidence that the scanner SAR was incorrect, or at least not limiting, at the time of the injury. Thus, the accuracy of scanner SARs reported by MRI scanners today, is questionable, and in any case, currently not easily verifiable independent of the scanner.
For example, and as shown in Table 2 below, some data provided from some preliminary investigations that compare scanner SARs with SARs measured from the time-derivative of the temperature response with fiber-optic probes in large gel phantoms placed in the scanner, confirm the suggestion that there is a range of error between the SAR projected using the scanner SARs versus the measured SAR.
TABLE 2Scanner vs. measured thermal SAR (W/kg), same phantomThermalFieldScannerSARrSAR1.5TGE21.341.5TGE21.81.5TSiemens2.41.61.5TSiemens21.41.5TPhilips44.21.5TPhilips43  7T(Philips)18.43.7
Indeed, if one seeks to independently measure deposited power using the above Eq. [1], access to the coil for power as well as QL and QU measurements is unavailable to scanner users, researchers, or medical physicists running routine checks on clinical units. Further problems arise with the testing of implanted devices, whose use in patients in need of diagnostic MRI grows exponentially. Devices that test safe at a specified scanner SAR level, may not be reliable if the scanner SAR includes unknown factors, or is scanner dependent as has been recently reported [Baker K B, Tkach J A, Nyenhuis J A, Phillips M, Shellock F G, Gonzalez-Martinez J, Rezai A R. Evaluation of specific absorption rate as a dosimeter of MRI-related Implant Heating. J. Magn. Reson. Imaging 2004; 20:315-320; Baker K B, Tkach J A, Phillips M, Rezai A R. Variability in RF-Induced Heating of a Deep Brain Stimulation Implant Across MR Systems. J. Magn. Reson. Imaging 2006; 24:1236-1242]. For example, a device testing as safe at a scanner SAR level of 4 W/kg, may in fact have only been tested at, say, 2 W/kg due to SAR being conservatively overstated. This is of particular concern, as there has been a large continuing increase in the number of patients with implanted medical devices (e.g., see FIG. 1A).
In summary: (i) if patients report burning sensations during MRI (see tabulation in FIG. 1B); or (ii) if internal device/implant makers wish to determine that their devices are safe at a certain SAR exposure level; or (iii) if questions arise during IRB assessment concerning the safe conduct of research studies, there are currently no scanner-independent means of determining whether the MRI machine is being operating within regulatory SAR guidelines, other than performing thermal testing of instrumented phantoms [Bottomley P A, Lardo A C, Tully S, Karmarker P, Viohl I. Safety and internal MRI coils. 2001 Syllabus. Special cross-specialty categorical course in Diagnostic Radiology: practical MR safety considerations for physicians, physicists, physicists and technologists. Oak Brook Ill.: Radiological Society of North America 2001; 85-90]. Such thermal testing is an involved and time consuming process.
It thus would be desirable to provide a new device or dosimeter for measuring or assessing SAR from RF deposition in MRI and methods for making such assessment using such a dosimeter. It would be particularly desirable to provide such a device and method that would allow such an assessment to be made that is scanner independent in comparison to prior art devices and techniques. Such collection devices preferably would be simple in construction and less costly to use in comparison to prior independent assessment methods and also would not require highly skilled users to utilize the device.